The present invention generally relates to optical semiconductor devices and more particularly to a semiconductor photodetection device used especially for fiber optics communication system.
FIG. 1 shows in general the structure of a conventional semiconductor photodetection device 10 of the type that receives incoming optical signal at a substrate surface side.
Referring to FIG. 1, the semiconductor photodetection device 10 is constructed on a substrate 11 of n-type InP, and includes a layer structure having an n-type InGaAs optical absorption layer 12 with low carrier concentration formed on the substrate 11 and a cap layer 13 of n-type InP formed on the layer 12. A p-type InGaAs region 16 and p-type InP region 15 are formed in the InGaAs optical absorption layer 12 and the n-type InP cap layer 13 by introducing a p-type impurity through an opening, which has been patterned in a dielectric protection layer 14 formed on the cap layer 13. An n-type electrode 17 is formed on the n-type InP substrate 11 and a p-type contact electrode 18 is formed on the p-type InP region 15, respectively. In the n-type electrode 17 is formed an optical window, through which an optical signal passes. In this illustrated embodiment, an antireflection film 19 is formed at the optical window on the substrate 11.
In operation of the photodetection device 10 shown in FIG. 1, a reverse bias voltage is applied between the electrodes 17 and 18. Under this condition, an optical signal having a wavelength of 1260-1620 nm used for fiber optics communication enters into the substrate 11 through the optical window. Because the substrate InP layer 11 is transparent to the light having the above wavelength, the incident signal light reaches the InGaAs optical absorption layer 12 without being absorbed by the substrate 11, and there occurs excitation of photocarriers in the optical absorption layer 12.
The frequency response of such a semiconductor photodetection device is generally determined by a time constant CR and a transit time of the carrier excited by the incident light, where C is a capacitance and R is an internal resistance of the device. In order to improve the frequency response of the semiconductor photodetection device 10, the time constant needs to be shortened and the carrier transit time also needs to be shortened. Because the carrier transit time increases proportionally to the thickness of the InGaAs optical absorption layer 12, it should be reduced in thickness, as much as possible in order to shorten the carrier transit time to improve the frequency response.
However, if the thickness of the InGaAs optical absorption layer 12 is reduced with the aim of achieving high speed, the optical absorption layer 12 can not absorb the incident light sufficiently, which degrades the quantum efficiency of the optical absorption.
Thus, because there exists a trade-off relationship between the frequency response and the quantum efficiency, it is difficult to obtain the optimum thickness of the InGaAs optical absorption layer 12 when designing semiconductor photodetection devices requiring high-speed response.
In order to solve this efficiency problem, in the conventional semiconductor photodetection device 10 of the substrate-side incident type shown in FIG. 1, the signal light that has not been absorbed by the optical absorption layer 12 is reflected by the p-type contact electrode 18 and re-introduced into the optical absorption layer 12 through the InP cap layer 13 to avoid the reduction in the quantum efficiency.
In the semiconductor photodetection device 10 shown in FIG. 1, while a metal layer constituting the contact electrode 18 is vapor-deposited on the n-type InP cap layer 13, heat resulting from the vapor-deposition forms an alloy metal layer at the interface between the InP cap layer 13 and the metal contact electrode 18. As a result, the planarity of the interface between the InP cap layer 13 and the contact electrode 18 is degraded. This degradation of planarity significantly lowers the reflectivity of the interface and reduces the amount of the signal light reflected by the interface, and therefore the signal light is mostly scattered by the interface and cannot be absorbed well enough in the InGaAs optical absorption layer 12. Consequently, the quantum efficiency is lowered and the amount of light returning from the photodetection device 10 to an optical fiber is increased, resulting in lower the transmission characteristics of fiber optics communication system.
In order to deal with the above mentioned problem, there has been a proposal, in Japanese Laid-Open Patent Publication 5-218488, that a dielectric layer 20 be interposed between the cap layer 13 and the contact electrode 18 as shown in FIG. 2, and would inhibit the alloying reaction between the metal layer of the contact electrode 18 and the InP layer of the cap layer 13. Similar or the same parts in FIG. 2 corresponding to the previously described parts in FIG. 1 are designated by the same reference numerals and the description thereof will be omitted.
This conventional structure of the substrate side incident type semiconductor photodetection device shown in Japanese Laid-Open Patent Publication 5-218488, however, has suffered from problem in that the contacting area between the contact electrode 18 and the InP cap layer 13 is reduced because of the dielectric layer 20, and the adherence between the metal layer of the contact electrode 18 and the dielectric layer 20 is not strong enough. Therefore, there has been a problem that the contact electrode 18 peels off during the manufacturing process, wire bonding process or flip chip mounting process.